Method and architecture to calibrate read operations in synchronous flash memory

ABSTRACT

Architecture to calibrate read operations in non-volatile memory devices. In one embodiment, a synchronous flash memory is disclosed. The synchronous flash memory includes a read sense amplifier, a verification sense amplifier, a switch, and an output buffer. The switch alternates electrical connection of the output buffer with the read sense amplifier and the verification sense amplifier. By measuring the distributions of voltage thresholds of erased cells versus voltage thresholds of programmed cells, differences in offsets between read state and write state of memory cells are determined. A specific margin is determined to ensure proper reads of the memory cells.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to non-volatile memory devicesand in particular, to the calibration of read and write operations ofsynchronous flash memory devices.

BACKGROUND OF THE INVENTION

Memory devices are typically utilized as internal storage areas inintegrated circuit devices. There are several different types of memory.One type of memory is random access memory (RAM). RAM has traditionallybeen used as main memory in a computer environment. A related memory issynchronous DRAM (SDRAM), which uses a clock pulse to synchronize thetransfer of data signals throughout the memory to increase the speed ofthe memory.

By contrast, read-only memory (ROM) devices permit only the reading ofdata. Unlike RAM, ROM cannot be written to. An EEPROM (electricallyerasable programmable read-only memory) is a special type ofnon-volatile ROM that can be erased by exposing it to an electricalcharge. Like other types of ROM, EEPROM is traditionally not as fast asRAM. EEPROM comprise a large number of memory cells having electricallyisolated gates (floating gates). Data is stored in the memory cells inthe form of charge on the floating gates. Charge is transported to orremoved from the floating gates by programming and erase operations,respectively.

A synchronous flash memory has the ability to read several thousandcells at a time, as contrasted to 16 cells at a time in a typicalstandard flash device. High read speeds of less than 10 nanoseconds arepossible with synchronous flash devices, making the devices comparablein speed to SDRAM. But unlike SDRAM, synchronous flash has a slow writespeed, typically about 10 microseconds. The slow write speed ofsynchronous flash is due primarily to the high voltage transistors usedin the write path. The high voltage transistors tend to be large, whichadds capacitance to the path. This capacitance significantly slows theread process.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora non-volatile memory device to increase operating performance whilemaintaining proper operation.

SUMMARY OF THE INVENTION

The above-mentioned problems with non-volatile memory devices and otherproblems are addressed by embodiments of the present invention, and willbe understood by reading and studying the following specification.

In one embodiment, a synchronous flash memory is disclosed. Thesynchronous flash memory includes a read sense amplifier, a verificationsense amplifier, a switch, and an output buffer. The switch alternateselectrical connection of the output buffer with the read sense amplifierand the verification sense amplifier. By measuring the distributions ofvoltage thresholds of erased cells versus voltage thresholds ofprogrammed cells, differences in characteristics (offsets) between readstate and write state of memory cells are determined. Thus, for a givensensing circuit, a specific margin is determined to ensure proper readsof the memory cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the read and write paths of theprior art.

FIG. 2 is a block diagram of one embodiment of a flash memory havingseparate read and write paths, according to the teachings of thisinvention.

FIG. 3 is a block diagram of the read and write path of one embodimentof a flash memory, according to the teachings of this invention.

FIG. 4 is schematic-block diagram illustrating how the bit lines of ablock of memory are coupled to the Y multiplexer and the latch/senseamplifier of one embodiment of a flash memory, according to theteachings of this invention.

FIG. 5 is a block diagram illustrating an erase verify path of oneembodiment of a flash memory, according to the teachings of thisinvention.

FIG. 6 is a block diagram of a system of calibrating read and writeoperations of one embodiment of a flash memory, according to theteachings of this invention.

FIG. 7 is a graph illustrating a distribution of erased-state voltagesand a distribution of programmed-state voltages of one embodiment of aflash memory, according to the teachings of this invention.

FIG. 8 is a schematic diagram illustrating an example prior art senseamplifier circuit coupled to a first bit line and a second bit line,according to the teachings of this invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of present embodiments, referenceis made to the accompanying drawings that form a part hereof, and inwhich is shown by way of illustration specific embodiments in which theinventions may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that logical, mechanical and electrical changes may be madewithout departing from the spirit and scope of the present invention.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the claims and equivalents thereof.

Referring to FIG. 1, a simplified block diagram illustrating a prior artread and write path of flash memory is shown. As shown, the read andwrite paths are coupled to memory array 132 by Y multiplexer (Y mux)110. The write path receives data from an input/output (I/O) connection144. The write path includes an input buffer (IB) 116 and a drivercircuit (D) 142. The driver circuit 142 is used to increase the voltagelevel applied to bit lines 114-1 to 114-m coupled to cells within thememory array 132. For example, a 3 volt signal from the input/outputconnection 144 is increased by the driver circuit 142 to approximately5.5 volts. The 5.5 volts is then applied to a drain of a respective cellin the memory array 132 when a voltage of approximately 10 volts isapplied to the gate of the memory cell to program the memory cell. The Ymux 110 is coupled between the driver circuit 142 and the memory array132 to selectively couple the program voltage, i.e. the 5.5 volts, to anaddressed cell. The Y mux 110 is also shown coupled to a state machine106. The state machine controls memory operations. In particular, thestate machine 106 is coupled to control the operations of the Y mux 110.That is, to direct the Y mux 110 to select a specific bit line.

The read path includes a sense amplifier (SA) circuit 130 coupled toreceive data from the Y mux 110 and an output buffer 118. The senseamplifier circuit 130 comprises a plurality of sense amplifiers that areused to read the cells in the memory array 132. As stated above, atypical sense amplifier circuit 130 may include 16 sense amplifiers.Generally, in order to select a given line, the Y mux 110 (decoder) isformed with two buses, each with 16 lines to select among a total of 256bit lines 114-1 to 114-m. The bit lines 114-1 to 114-m are clusteredinto groups of 16 lines each. There are 16 such groups. The statemachine 106 is coupled to an output of the sense amplifier circuit 130to monitor the output of the sense amplifier circuit 130.

Referring to FIG. 2, a simplified block diagram of a synchronous flashmemory 200 of an embodiment of the present invention is illustrated. Theflash memory 200 is shown having control circuitry 208 to control memoryoperations to a memory array 232. Such memory operations includereading, writing and erasing. The control circuitry is illustrated ascontaining command execution logic 204 and a state machine 206. Thestate machine 206 is commonly referred to as the specific device thatcontrols the memory operations. The synchronous flash memory 200 is alsoshown having an address register 214, a row counter 220, a row or Xdecode circuit 224, a bank decode 226, a voltage pump 240 and an inputbuffer 216. The voltage pump 240 is used to increase the voltage levelsduring read, write and erase operations. The X decode circuit 224 isused to decode address request to rows of memory cells in memory array202-1 to 202-k. The bank decode 226 is used to decode address requestsamong the banks 202-1 to 202-k of memory in the memory array 232.Although the synchronous flash memory embodiment described has fourbanks of memory, it will be understood in the art that the synchronousflash memory 200 could have more than four or less than four memorybanks and the present invention is not limited to four banks of memory.

The synchronous flash memory 200 of FIG. 2 is illustrated as also havinga Y mux/decoder 210 and write path isolation circuit 212. The Ymux/decoder 210 is used to decode and multiplex address requests tocolumns of memory cells in the memory array 232. The write pathisolation circuit 212 decouples the Y mux/decoder 210 from the memoryarray 232 during read operations. Also illustrated in FIG. 2, is alatch/sense amplifier circuit 230, a read path isolation circuit 228, aFIFO circuit 222, and an output buffer 218. The latch/sense amplifiercircuit 230 is coupled to read addressed or accessed memory cells in thememory array 232. The read path isolation circuit 228 decouples thelatch/sense amplifier circuit 230 from the memory array 232 during writeoperations. A processor 250 is shown coupled to the synchronous flashmemory 200 to provide external commands, address requests and data tothe synchronous flash memory 200.

Referring to FIG. 3, a block diagram of the read and write paths of oneembodiment of the present invention is illustrated. As illustrated, thewrite path comprises input buffer (IB) 316 coupled to an input/outputconnection (I/O) 344 to receive data. A driver circuit (D) 342 iscoupled to the input buffer 316 to drive a program voltage(approximately 5.5 volts) when programming a cell. Y mux 310 is coupledto the driver circuit 342 to direct the program voltage to a selectedbit line 314-0 to 314-n. Write path isolation circuit 312 is coupledbetween the Y mux 310 and memory array 332 to selectively decouple the Ymux 310 from the bit lines 314-0 to 314n of the memory array 332 duringread operations. Referring back to FIG. 2, the write path isolationcircuit 212 is coupled to control circuitry 208, wherein the controlcircuitry 208 selectively activates the write path isolation circuit212.

The read path of FIG. 3 includes a read path isolation circuit 328 thatis coupled via bit lines 314-0 to 314-n to an end of the memory array332 opposite the Y mux 310. Referring back to FIG. 2, the read pathisolation circuit 228 is coupled to the control circuitry 208, whereinthe control circuitry 208 selectively activates the read path isolationcircuit 222. As illustrated in FIG. 3, a latch/sense amplifier circuit330 is coupled to the read path isolation circuit 328 by the bit lines314-0 to 314-n. In a read operation, a page of memory cells are read atthe same time. That is, all the cells coupled to a word line areactivated at the same time thereby dumping their contents into thelatch/sense amplifier circuit 330 at the same time. Therefore, thelatch/sense amplifier circuit 330 must contain a latch/sense amplifierfor each bit line 314-0 to 314-n. FIFO circuit 322 is coupled to thelatch/sense amplifier circuit 330 to control the flow of data from thelatch/sense amplifier circuit 330. As shown, output buffer 318 iscoupled between the FIFO circuit 322 and the input/output connection344.

Once the cells are coupled to a word line, latch/sense amplifier circuit330 decodes the data which can be randomly read within the page. To beable to obtain this speed, transistors within the latch/sense amplifiercircuit 330 require a thin oxide layer, such as of approximately 70 Å,with short channel lengths. Therefore, the Y-mux of a typical flashmemory having an oxide layer of 200 Å and a relatively long channellength will not work effectively in synchronous flash memory read paths.In effect, the present invention uses a first multiplexer 310 (the Y mux310) in the write path and a second mux 330 (the latch/sense amplifier330) in the read path. This allows the write path to use relatively highvoltage transistors in Y mux 310 to provide the voltage needed toprogram cells and the read path to use higher performance lower voltagetransistors in latch/sense amplifier circuit 330 in reading the cells.

Referring to FIG. 4, a schematic-block diagram is shown, illustratinghow a first mux 410 and a second mux 430 are coupled to bit lines 414-0to 414-n of one embodiment of the present invention. As shown, the firstmux 410 (Y mux 410) is positioned at a first end of memory array 432 andcoupled to a first end of bit lines 414-0 to 414-n. More specifically,write path isolation circuit 412 is coupled between the first mux 410and the first ends of the bit lines 414-0 to 414-n. The second mux 430(or latch/sense amplifier circuit 430) is positioned at a second end ofthe memory array 432 and coupled to a second end of bit lines 414-0 to414-n. More specifically, the read path isolation circuit 428 is coupledbetween the second mux circuit 430 and the second ends of the bit lines414-0 to 414-n. FIG. 4 also illustrates how cells 458 are coupled to thebit lines 414-0 to 414-n and word lines 415-0 to 415-q. Source line 438allows a bias voltage to be applied to erase the cells.

Although FIG. 4 illustrates the first multiplexer 410 being coupled tothe first end of the bit lines 414-0 to 414-n and the second multiplexer430 being coupled to the second end of the bit lines 414-o to 414-n, itwill be understood in the art that the present invention is not limitedto such. The first mux 410 and the second mux 430 may be coupled to thebit lines 414-0 to 414-n in another manner. For example, the first mux410 and the second mux 430 may both be coupled to the same end of thebit lines 414-0 to 414-n. In addition, as understood in the art, theterm mux or multiplexer as used in the present invention is also used todescribe a decoder to couple selected inputs with selected outputs.

In another embodiment, an erase verify path is coupled to the writepath. This embodiment is illustrated in FIG. 5. The erase verify path isused to verify that cells in a block of memory in the memory array 532are erased after an erase pulse has been applied to the block. Sinceverification of the cells in a block of memory is performed on a limitednumber of cells, the high performance transistors of the second mux 530(latch/sense amplifier 530) are not required. Accordingly, the first mux510 (Y mux 510) may be used. As illustrated in FIG. 5, the erase verifypath includes a sense amplifier circuit 560 to read the memory cells.Control circuitry 508 determines if another erase pulse should beapplied to the block of cells being erased.

Referring to FIG. 6, a block diagram illustrates a system of calibratingread and write operations of one embodiment of a flash memory. In oneembodiment of the invention a distribution of voltage thresholds andtiming thresholds of memory cells can be determined to properlydistinguish the program states and erase states of the memory cellsduring read and erase operations. This improves reliability of the readand erase operations, allows for trimming of the voltage thresholds andtiming thresholds of the memory cells to reduce differences of thosethresholds.

A verification sense amplifier 660 is coupled to an output buffer 618via a switch 648. A read sense amplifier 630 is also coupled to theoutput buffer 618 via the switch 648. The switch 648 alternateselectrical connection of the output buffer 618 with the verificationsense amplifier 660 and the read sense amplifier 630. The output buffer618 is coupled to an output circuit 644.

Differences in characteristics (offsets) between read sense amplifier630 and write sense amplifier are determined by measuring thedistribution of voltage thresholds of erased cells versus voltagethresholds of programmed cells. If a bias voltage is applied to a gateof a programmed cell, the bias voltage is insufficient to overcome thethreshold voltage of the programmed cell. The programmed cell will notconduct and thus be unaffected by the bias voltage. If the bias voltageis applied to an erased cell, the bias voltage overcomes the thresholdvoltage of the erased cell. The erased cell conducts, allowing theerased cell to be programmed. The gate bias voltage remains common forthe entire array. The sense amplifier circuitry, however, may havedifferences in operation based on fabrication, environmental conditions,and layout. As a result, some memory cells may be read differently bythe two sense amplifiers. This operation problem is mainly experiencedin marginal memory cells. That is, a memory cell that is verified asbeing programmed may be close to an acceptable margin. When this cell isread by a different sense amplifier, the cell may read as being erased.As explained below, the sense amplifier voltages and timing can beadjusted to compensate for calibration differences.

The margin test mode of the array 632 is provided through either theverification path or the read path. This allows statistical analysis tobe taken of the threshold distribution of the memory cell array array632 using an external tester. The statistical analysis is taken of bothread operations and verification operations to determine if offsetsexist. If offsets are detected, decisions can be made regarding trimlevels to provide proper margins for the read path verification. Forexample, trigger timing for the sense amplifier circuitry can beadjusted to compensate for measured offsets between the sensingcircuits.

Referring to FIG. 7, a graph illustrates a distribution of erased-statevoltages 770 and a distribution of programmed-state voltages 780 of oneembodiment of a flash memory. A cell margin is a voltage differentialbetween maximum erased state voltage 772 (V_(E-MAX)) and minimumprogrammed state voltage 776 (V_(P-MIN)). The cell margins can beaffected by changes in bit line capacitances and cell currents due toprocess or layout variations, temperature, and other factors. Thus,variations in the bit line capacitances affect the charge shared voltagelevel, and variations in cell current affect the voltage differentialthat is established. As stated above, gate voltage 774 remains commonfor both read paths. The read paths and sensing circuitry, however,change between paths. These variables in the sensing path defineV_(E-MAX) 772 and V_(P-MIN) 776. That is, for a given sensing circuit, aspecific margin is needed to insure proper reads. When read senseamplifier 630 is used, erase distribution 770 (established with verifycircuit 660) may appear as having offset 778. As such, the circuits arenot calibrated. Verify circuit 660 can be adjusted so that all erasedcells remain below V_(E-MAX) when read using circuit 630.

Referring to FIG. 8, a schematic diagram is shown, illustrating anexample sense amplifier circuit 890 coupled to a first bit line 891 anda second bit line 892. The sense amplifier circuit 890 includes across-coupled NMOS transistor pair 893-1 to 893-2, and a cross-coupledPMOS transistor pair 894-1 to 894-2. A first common node 895 is coupledto the cross-coupled NMOS transistor pair 893-1 to 893-2. A secondcommon node 896 is coupled to the cross-coupled PMOS transistor pair894-1 to 894-2. The first common node 895 and the second common node 896apply adjustable trigger voltages to the cross-coupled NMOS transistorpair 893-1 to 893-2 and the cross-coupled PMOS transistor pair 894-1 to894-2. The timing and voltage levels of nodes 895 and 896 can bemodified to adjust the sensitivity of the amplifier. Implementing thesense amplifier of FIG. 8 as sense circuits 660 and 630 allows oneembodiment of the present invention to be calibrated for minor offsets.

CONCLUSION

A non-volatile memory device having a method and architecture tocalibrate its read and write operations has been disclosed. In oneembodiment, a flash memory device has a memory array, a firstmultiplexer and a second multiplexer. The memory array has non-volatilememory cells arranged in columns and rows. Each memory cell in a columnis coupled to an associated bit line. The first multiplexer is coupledto select bit lines during write operations to the memory array. Thesecond multiplexer is coupled to select bit lines during read operationsfrom the memory array. Both verify and read paths can be coupled tooutput circuitry to allow a statistical analysis to be performed.Offsets, therefore, can be corrected between the paths.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiments shown. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is intended that this invention be limited onlyby the claims and the equivalents thereof.

1. A flash memory device comprising: a memory array having erasableblocks of memory cells, each block of memory cells being arranged in arow and column configuration, wherein each column of memory cells iscouplable to an associated bit line; control circuitry to control memoryoperations to the memory array; a verify sense amplifier to verify aprogram state of the memory cells, the verify sense amplifier is coupledto the associated bit lines; a read sense amplifier to read a programstate of the memory cells, the read sense amplifier is coupled to theassociated bit lines; and a switch to selectively couple either theverify sense amplifier or the read sense amplifier to an output circuitwherein the verify sense amplifier and the read sense amplifier haveadjustable sensitivity.
 2. The flash memory device of claim 1 whereinthe flash memory is a synchronous flash memory.
 3. The flash memorydevice of claim 1 wherein the verify sense amplifier comprisestransistors with a gate oxide of approximately 200 Å and the read senseamplifier comprises transistors with a gate oxide of approximately 70 Å.